User:Robertinventor/O2 solubility in Martian near-surface environments and implications for aerobic life - notes

These are notes to help with background research for the article: Cold salty water on Mars may have enough oxygen for air breathing microbes, possibly even simple animals like sponges

It is a theoretical rather than an experimental study, based on previously published research about the properties of salty brines on the surface of Mars. The relevance of the salts is that they let the brines remain liquid at very low temperatures as low as 140 K, that's -133 C. Even though there is little oxygen in the atmosphere, such low temperatures make it easier for it to enter the water.

Some highlights from my notes
Some highlights from my notes, many of them not mentioned in the articles about their research or in the press release:


 * 6.5% of the Mars surface, current Mars, would be able to support sponges with supercooling (a process that has been confirmed in experiments with brines mixed with regolith - the brines stay liquid right down to -133 C).


 * Some areas even would have oxygen levels the same as for modern Earth oceans


 * Mars has been tilted optimally for oxygen in the brines for the last 5 million years and will continue like that for at least another 10 million years.


 * When the axial tilt is more than 45 degrees (which happens with Mars occasionally, leading to equatorial ice sheets and ice free poles) then the levels of oxygen are not high enough for sponges.


 * For the last 20 million years there has always been enough oxygen for sponges according to their models, if the brines are there


 * Modern Mars would have more oxygen in its brines than early Earth did before the Great Oxygenation event


 * The oxygen in these brines may explain some weathering processes on Mars and even the methane seeps detected by Curiosity.


 * They only studied processes for the top few cms of the Mars surface and oxygen mixing with water in those layers (below that, then the ground would be permanently frozen except at geological hot spots or subglacial lakes.


 * They do not speculate about subglacial lakes so this is speculation by commentators on the article, not by the authors themselves and their research would not seem to apply directly to that situation so I think we shouldn't mention it, or if we do, as a question, not a suggestion.

Importance of oxygen for life
They open out by talking about why aerobic life (i.e. based on oxygen) is important

"Aerobic respiration is the most widespread and energetically favourable metabolism on Earth; it enabled complex multicellularity."

Existing research on brines
They then summmarize existing research on brines on Mars and on the oxygen levels needed for oxygen metabolism microbes (a millionth of a mole per cubic meter) and for primitive sponges (0.02 moles of oxygen per cubic meter).

Combining brine studies with a simplified general circulation model for Mars
They take this experimental and theoretical work about the brines and they combine it with a general circulation model for the Mars atmosphere. In this model they use temperatures averaged out throughout the day and year, ignoring seasonal and diurnal variations. So it is rather a simplified model but a first look at the situation there:

"To address these questions, first, we develop a comprehensive thermodynamic framework applicable to Martian conditions that calculates the solubility of O2 in liquid brines composed of water and salts, including perchlorates (Ca(ClO4)2, Mg(ClO4)2, NaClO4), chlorides (MgCl2, NaCl) and sulfates (MgSO4). The supply of O2 for our calculation is the atmosphere, and hence, our approach is valid for the surface and shallow subsurface (‘near surface’) only, where brines are assumed to communicate with the atmosphere.

Second, we couple this solubility framework to a Mars general circulation model (GCM)23,24 to compute the solubility of O2 as a function of annually averaged values of pressure and temperature varying with location on Mars today (for an obliquity of ~25°). Using annual average climate values precludes the specifics of diurnal and seasonal variations of these aerobic environments, but gives a concise first look into the regions that are most or least likely to sustain high O2 solubilities."

For the brines they go into a lot of theoretical detail about how they work out the solubility of oxygen into a brine taking account of the temperture and the various anions and cations in the water. This is theoretical, but it is also in agreement with experimental data - basically this section is justifying their model. I won't go into it in these notes. We can assume they got it right as it is straightforward non controversial chemistry and physics, and it passed peer review.

Eutectic mixtures and supercooling
They then look at eutectic mixtures and supercooling. Eutectics describe how if you mix salts with water then some of the salt may stay out of solution but the solution will automatically evolve to the optimal concentrations to stay liquid at the lowest possible temperatures as you cool it down. Salt moves in or out of solution as you cool it to keep it optimal.

Then with supercooling it can get even colder. Down to 140 K or -133 C. This is a quote from that section of the paper:

"Next to having very low eutectic temperatures, Mg- and Ca-perchlorate brines can exist as liquids in a supercooled state far below their eutectic temperature, as shown by experiments (~209 K and ~198 K, respectively), to ~140–150 K under modern Martian conditions"

They then give evidence that eutectic mixtures can form on Mars and may for instance be present in the recurring slope lineae - and that even when mixed with regolith (as they would be) can still be supercooled down to that 140 K or -133 C.

"Aqueous environments, in the form of brines, can exist today at, and especially below, the surface despite the thin atmosphere and overall cold climate. Recent evidence demonstrates hydrated magnesium (Mg) and calcium (Ca) perchlorate salts at various locations on the Martian surface and shallow subsurface12–15, which indicate the existence of Mg(ClO4)2-Ca(ClO4)2-H2O brines, and which could, in some cases, be associated with flow structures such as the modern recurring slope lineae. Ca- and Mg-perchlorate brines exhibit a much lower freezing point than pure water, by as much as 60–80 K (refs 16,17), and at their eutectic composition, they effectively supercool down to 140–150 K before transitioning into a glass, even when mixed with Martian regolith simulant16. Calculations18 and experiments16 using brines containing Martian soil simulants with perchlorates (for example, an ana-logue of the Mars Phoenix landing site soil18) show that dissolved perchlorate concentrations can readily reach eutectic concentrations during freezing."

Levels of oxygen enough for microbes everywhere - levels for sponges with supercooling near the poles and can even reach concentrations typical of sea water
They do two versions of their calculations, with, or without supercooling. In both calculations they find that all the brines on Mars, no matter where they are, can potentially have oxygen levels high enough for microbes that use oxygen.

"We find that, on modern Mars—accounting for all uncertainties, for our best estimate and the worst case, both with and without super-cooling and also for temperatures above 273 K where our solubility model has been validated—the solubility of O2 in various fluids can exceed the level required for aerobic respiration of ~10−6 mol m−3 for microbes by ~1–6 orders of magnitude."

The question then is whether it can ever reach the 0.02 moles per cubic meter needed for sponges. They find that without supercooling it never reaches high enough concentrations for this. The best value at the south pole, without supercooling, is about a five thousandth of a mole per cubic meter (from the colour coding of their Figure 1 b)

The text summary says

"For our best estimate, but without supercooling, liquid perchlorate brines can occur primarily at equatorward latitudes below 50° and − 50°, with differences in potential O2 solubility of one order of magnitude, depending on the specific location (Fig. 3b).

Even for the worst case (with and without supercooling), values for the solubility of O2 in Ca- and Mg-perchlorate near-surface brines are generally 1–2 orders of magnitude above 10−6 mol m−3 (Figs. 2d,e and 3d)."

With supercooling they find that there are many oases where the oxygen levels on present day Mars can rise to levels high enough for sponges. There the thing about sponges is that they are made up of fine networks of sheets of cells. The salty water flows in and out of the sponge and the thin layers only one cell thick have salty oxygenated water passing by them on either side. So a small sponge a few mms in size can take in oxygen from the water rather readily. They cite a paper that says this can happen at 0.02 moles of oxygen per cubic meter. Not only that, the oxygen levels can even reach levels comparable to Earth's oceans today.

"For our best estimate (including supercooling), the results dis-play large gradients in O2 solubility for Ca- and Mg-perchlorates across Mars, with polar regions having the greatest potential to harbour near-surface fluids at 2 × 10−1 mol m−3 of dissolved O2, and the  least O2-rich environments in the tropical southern highlands  at ~2.5 ×   10−5 mol m−3 of dissolved O2. The O2 solubility of near-surface brines across Mars today could vary by five orders of magnitude (Fig. 3a). This trend results from lower temperatures at higher latitudes promoting O2 entry into brines."

There the highest value there they give, 0.2 moles per cubic meter of dissolved O2 in polar regions, is comparable to the levels in modern oceans on Earth.

MY COMMENT HERE: Remarkable! Modern fish would in principle get enough oxygen to live there - if they could withstand the perchlorates and the ultra cold temperatures at night of course, which modern Earth life couldn't do.

6.5% of the Martian surface could support enough oxygen for primitive sponges in the oxygenated brines
They find that 6.5% of the surface of present day Mars could have oxygen levels high enough for aerobic respiration by primitive sponges:

"Moreover, for supercooled Ca- and Mg-perchlorate brines on Mars today, ~6.5% of the total Martian surface area could support far higher dissolved O2 concentrations—enabling aerobic oases at dissolved O2 concentrations higher than 2 × 10−3 mol m−3, sufficient to sustain the respiration demands of more complex multicellular organisms such as sponges (Figs. 2 and 3). Such aerobic oases are common today at latitudes poleward of about 67.5° and about − 72.5° (Fig. 3a)."

This is where I assume they mean that the brines absorb the oxygen while supercooled and then retain it as they warm up to more habitable temperatures because the temperatures they consider, -133 C are very low, usually the limit for Earth life is -20 C. Alternative biochemistries could get to much lower temperatures by using a metabolism with perchlorates inside the cells, and hydrogen peroxide but they don't have anything to suggest they are talking about such biochemistries here.

Better for oxygen than early Earth before the great oxygenation event
They make a comparison with Early Earth before the "Great Oxygenation Event" that added oxygen to our atmosphere, and they find that Modern Mars, very surprisingly, will have better oxygenated brines than early Earth. Mainly because of the cold conditions there and the way that the perchlorates can be liquid at such low temperatures.

"By comparison, modern Mars enables greater O2 solubilities due to its much colder surface temperatures in relation to early Earth and a small but relevant amount of atmospheric O2. Thus, in principle, Mars could offer a wide range of near-surface environments with enough dissolved O2 for aerobic respiration like that seen in diverse groups of terrestrial microorganisms."

Chemical weathering
They comment that at the levels they calculated, oxygen in the brines could contributed to the chemical weathering of rocks, and also the release of methane, both surprising recent and hard to explain observations on Mars.

"At the solubilities we find, O2 could play a role in chemical weathering of Mars’s crust11,36 and may help to explain a range of surprising geochemical, geological and atmospheric observations, including the presence of highly oxidized phases in Martian rocks9–11 and seasonal variability of methane at Gale crater37."

Mars' axis is currently optimally tilted for oxygen rich brines
They find that Mars currently has the potential for far higher oxygen levels than normal. For the last 5 million years they would have been particularly oxygen rich. The 15 million years before then would have seen oxygen levels 200 times lower than they are today. This is due to changing tilt of the Mars axis (our axis is stabilized by the Moon while Mars soemtimes tilts so far, beyond 45 degrees tilt, that it has equatorial ice sheets and ice free poles).

"Integrating these results with obliquity variation through time22, it is apparent that the past 5 Myr have supported particularly O2-rich environments (Fig. 4c), while the preceding 15 Myr favoured average maximum O2 solubility values in Mg- and Ca-perchlorate brines up to ~200 times lower than today."

They show that when tilted by more than around 45 degrees - when Mars loses its polar ice caps and its atmosphere becomes much thicker, then the oxygen levels are no longer high enough for sponges. However this hasn't happened in the last 20 million years. Projecting forwards, it should continue at its current high levels similarly to the last 5 million years for at least 10 million years into the future (from their figure 4).

They conclude that this is a new way to get levels of oxygen on a planet that are high enough for aerobes, without the need for photosynthetic life to create it.

"On Earth, aerobic respiration appears to have followed in the evolutionary footsteps of oxygenic photosynthesis, reflecting the scarcity of O2 on Earth before photosynthesis. However, by sourcing O2 in a different way, Mars shows us this need not be the case, broadening our view of the opportunities for aerobes on other planetary bodies."